You're going to end up with some level of IDB'd (involuntarily denied boarding) passengers in any world where seats/safety equipment break, equipment changes, crew members get sick and/or time out and airline personnel need to be shuttled to crew another flight that would otherwise be entirely cancelled, or unexpected weather [higher than typical temperatures, unfavorable winds] or airport conditions [runway closures/temporary shortening] preclude a full gross weight takeoff.
As a passenger, I appreciate that my airfares are lower and some airfares have increased flexibility because the airlines have a deep understanding of the turn-up ratio and sell tickets in light of that fact. I appreciate the cases where [probably without my awareness] a flight or cabin crew/member [or maintenance tech and part] has been last-minute flown in to crew/fix a flight that I ended up taking rather than having it be cancelled.
Does it suck to be IDB'd? Sure. Does it happen often? Almost never (around 23 in a million or 1 in 44K embarkations). People in the US are about 5.5 times more likely to be killed in a car crash in a given year than be IDB'd on a given flight.